Method for centralising a tight fitting casing in a borehole

ABSTRACT

Method for the centralization of drill casings in a tight borehole, both vertical and deviated, comprising the following operations: Application on the casing section in question, of an adherence strip equipped with one or more housings and rapid adjustable lever closure; Insertion of suitable moulds in the housings of which the strip is equipped; Injection into the moulds of plastic material characterized by a high mechanical resistance, a high degree of surface adhesion and resistance to abrasion by friction, and also in-place elasticity, higher than the elasticity of the casings; Removal of the adherence strip once the hardening of the plastic material in the shape of the mould, has been completed.

[0001] The present invention relates to a method for the centralizationof drill casings to be used for lean profile applications, whichsubstantially comprises the application of strips of ceramic materialonto the outer walls of the casing.

[0002] During the drilling of oil wells, set casing operations of thehole are effected, which comprise the positioning of specific casings atpre-established intervals in relation to the depth reached andcharacteristics of the formations penetrated.

[0003] When effecting set casing, in order to guarantee an easy loweringof the casings into the well, it is generally necessary to ensure thatthe difference between the external casing diameter and the holediameter (clearance) is maintained at sufficiently high values.

[0004] During drilling, according to conventional technologies, theclearance between casing and hole can vary from values of about 6-7inches (15-16 cm) for the surface and/or intermediate phases, up tovalues close to an inch for the deeper phases (for example: 26″ hole fora 20″ conductor pipe; 17½″ hole for a 13⅜″ casing; 12¼″ hole for a 9⅝″casing; 8½″ hole for a 7″ casing).

[0005] The necessity of adopting high clearance values is linked to thehigh rigidity of pipes with a greater diameter/thickness and to theimpossibility of drilling perfectly vertical holes and/or withcontrolled curvature. The poor flexibility, in fact, of the pipestogether with a certain degree of irregularity of the hole (deviations,tortuosity and/or squeezing), can make the lowering of the casings intothe well problematical, above all during the surface and/or intermediatephases.

[0006] The necessity, according to conventional technologies, ofeffecting holes with a much larger diameter with respect to that of thecasing, to guarantee its lowering to the well bottom during the surfaceand intermediate phases, causes the formation of a considerable quantityof waste products and a lengthy stay time of the plant on the territory,with consequent long drilling times and high costs.

[0007] The Applicant is holder of the copending Italian patentapplication MI 2000A 000007 of May 5, 2000 which relates to an improvedmethod for the drilling of oil wells and contemporaneous positioning ofspecific casings, characterized by maintaining the clearance more orless constant for the whole well depth desired, which consists ineffecting the drilling of the well section of interest, maintaining ahigh degree of regularity of the hole by the use of automatic equipmentfor verticality and/or curvature control.

[0008] This drilling method allows, with the same diameter of thecasing, the dimensions of the upper part of the well (surface andintermediate casings) to be reduced. This enables a substantialreduction in materials (sludge, cement and steel) and consequently inthe production of waste products. This technique also avoids thenecessity of producing holes with a diameter which is too high withrespect to the casings to be positioned, which generally implies thepossibility of effecting holes with a lower diameter with respect to theconventional technique. All of this has a positive influence on theadvance rate of the chisel bit and therefore allows a reduction in theproduction times of the holes with a consequent decrease in costs. Thetechnique according to said invention also enables significant savingsto be obtained on the operating costs, as the verticality and/or regularcurvature of the well facilitates all workover and well interventionoperations.

[0009] This method comprises a drilling and set casing technique whichallows the positioning of casings, operating with a clearance which ismore or less constant at values of about 1.5 inches (3-4 cm) for thewhole well depth desired. The fact of operating with a reduced clearanceallows, with the same number of casings and end-diameter of the casing,the dimensions of the upper part of the well to be significantlyreduced.

[0010] This solution is particularly suitable for applications in deep,vertical or off-line wells, also in the presence of formations with alow drillability and/or difficulty in trajectory control. In thesecases, on adopting the technique described, a considerable saving oftime and drilling costs is obtained.

[0011] In the drilling method mentioned above, as generally in all leanprofile applications, the centralization of the casings, when effectedwith traditional procedures, is highly critical.

[0012] This criticality is due to the lack of physical space necessaryfor the mechanical installation of centralization systems (reducedspaces corresponding to mechanical fragility).

[0013] As the inclination of the hole increases, so does the criticalitydue to the higher casing/hole wall interference, obviously greater inthe hole sections with the presence of more or less severe doglegs.

[0014] The absence of centralization of the casing, however, has thefollowing disadvantages:

[0015] Increase in the friction factor, with an increase in the wear ofthe casing and consequent decrease in its resistance properties;

[0016] Increase in the risks of the wedging of the casing with the riskof not succeeding in correctly lowering the casing to the bottom;

[0017] Unbalance in the flow by-pass area;

[0018] Reduction in the quality of cementations by the canalization ofcement during displacement.

[0019] For conventional (standard) well profiles, these problems aredrastically reduced by centralizing the casing.

[0020] The centralizing is, in fact, obtained by inserting along theouter surface of the casing, various elastic metallic blades (similar toleaf springs), with an extended diameter comparable to that of the hole,capable of keeping the casing at a distance from the wall and uniformlycentred with respect to the hole axis:

[0021] This solution cannot however be applied in the case of leanprofiles as:

[0022] All standard centralizers existing on the market have a bladesupporting body and a stop collar for attaching them to the pipe; thisincreases the overall bulk which excludes their application for systemswith reduced diameter clearances;

[0023] Centralizers for particular applications, such as for exampleintracasing centralizers, drastically modify the total rigidity of thecasing, which viceversa must be able to maintain a good elasticity(flexibility) for following the geometry of the hole to the utmost.

[0024] The Applicant has now found, according to the object of thepresent invention, that it is possible to overcome all the knowndrawbacks in the state of the art by effecting the centralization ofdrill casings by means of the application of strips of ceramic materialon the outer walls of the casings.

[0025] The object of the present invention therefore relates to a methodfor the centralization of casings for applications of the lean profiletype, both vertical and off-line, which comprises the followingoperations:

[0026] Application on the casing section in question, of an adherencestrip equipped with one or more housings and rapid adjustable leverclosure;

[0027] Insertion of suitable moulds in the housings of which the stripis equipped;

[0028] Injection into the moulds of plastic material characterized by ahigh mechanical resistance, a high degree of surface adhesion andresistance to abrasion by friction, and also elasticity in place, higherthan the elasticity of the casings;

[0029] Removal of the adherence strip once the hardening of the plasticmaterial in the shape of the mould, has been completed.

[0030] An important advantage of the method described above is thepossibility of its being effected directly on site, thus obtainingcentralizers whose shape, dimension, number and position can beestablished a priori, in relation to the operating requirements. Theposition, length and thickness with which the centralizers are to beproduced, can, in fact, be calculated and determined a priori, inrelation to the diameter of the casing, the length and inclination angleof the hole section along which the casing is to be lowered.

[0031] Once the project characteristics have been defined, the pipes areblasted to obtain a surface capable of ensuring correct adhesion of theresin. At the end of the blasting phase, each pipe is equipped with aspecific adhesion strip, obtained using a transparent plate made ofplastic material with a high resistance (FIG. 1), having a rapidadjustable lever closure, capable of housing die cast moulds: FIG. 2illustrates an example in which from a minimum of one to a maximum ofthree die cast moulds can be present.

[0032] Each mould has a specific injection hole, through which the resincompound, obtained by means of an appropriate mixing system betweenresin and hardener, is poured. The adhesion strip-mould system thusobtained allows, on the one hand, the centralizers to be correctlypositioned and distanced, both radially and axially, along the pipe; onthe other it guarantees the regular shape of the centralizers, keepingthem correctly in position until the binding reaction of thecatalyst/resin system has taken place. Once the binding time necessaryfor the hardening of the centralizers is over, it is possible to removethe adhesion strip and relative die cast shapers.

[0033] With reference to the figures enclosed with the presentinvention, which have the sole purpose of illustrating the objectaccording to the present invention,

[0034]FIG. 1 represents a detail of the transparent plate of plasticmaterial in which A and B represent countersunk bolts forming the rapidclosure to be used when the plate is wound around the casing and inwhich the housings for inserting the moulds can be observed.

[0035]FIG. 2 in turn illustrates the same plate indicating the rapidadjustable lever closures, under B, and in which there is a detail ofthe same closure, under C.

[0036] Said closure is also represented in FIG. 3, in which B representsdrawings thereof, for an embodiment in zincated steel, and A representsthe drawing of the hook for the adjustable lever closure.

[0037] Confirming the validity of what is specified above with respectto the method for the centralization of casings and the operating stepscomprised therein, we would like to stress again that there are twoelements which mainly characterize said method:

[0038] A compound of resins, characterized by a high mechanicalresistance, a high degree of surface adhesion and resistance to abrasionby friction and, finally, elasticity in place higher than the elasticityof the casings.

[0039] The production, also directly on site, of centralizers, with thepossibility of appropriately varying the shape, dimension, number andposition thereof, in relation to the operating requirements.

[0040] The resinous material naturally has a primary function; any knownmaterial can be suitable for the purpose provided it is characterized bythe appropriate qualities: as an example, high resistance epoxy resinscan be mentioned.

1. A method for the centralization of drill casings for applications ofthe lean profile type, either vertical or off-line, comprising thefollowing operations: Application on the casing section in question, ofan adherence strip equipped with one or more housings and rapidadjustable lever closure; Insertion of suitable moulds in the housingsof which the strip is equipped; Injection into the moulds of plasticmaterial characterized by a high mechanical resistance, a high degree ofsurface adhesion and resistance to abrasion by friction, and alsoelasticity in place, higher than the elasticity of the casings; Removalof the adherence strip once the hardening of the plastic material in theshape of the mould, has been completed.
 2. The method for thecentralization of drill casings for lean profile applications accordingto the previous claim, wherein the adhesion strip contains a maximum ofthree housings.
 3. The method for the centralization of drill casingsfor lean profile applications according to claim 1, wherein the plasticmaterial injected into the moulds is preferably an epoxy resin.